Major depressive disorder (MDD) is a condition that affects an alarmingly high number of people at some point during their lives, with a prevalence varying between 3% and 16%, depending on the country. Although a number of pharmaceutical interventions are available, some MDD sufferers are partially or completely resistant to these treatments, and it is estimated that up to 50% of patients do not achieve full remission. In addition, anti-depressant drugs have a number of side-effects, including nausea, insomnia and weight gain. Alternative and/or complementary forms of treatment for MDD are therefore needed. Additionally, it is desirable to develop methods to modulate a mood aspect or parameter of an individual without reliance on pharmaceutical interventions.
A substantial body of research has been devoted to the study of facial movements as they relate to particular emotions or moods. Facial expressions for basic emotions (happiness, fear, surprise, etc.) have been found to be well-defined and universal across cultures. Certain facial muscle movements can be easily controlled voluntarily, while others occur primarily during “genuine” emotions. For example, voluntary smiles (e.g. smiles for social purposes, without any particular emotional involvement) usually consist only of the upward curving of the lips, whereas spontaneous smiles due to positive genuine spontaneous emotions also involve the eyes. The specific pattern of eye movement associated with genuine smiling is known as the Duchenne marker, and is characterized by a raising of the cheeks and the appearance of crows-feet wrinkles next to the eyes. Different neural pathways mediate these two types of smiles.
Voluntary smiles are initiated in the motor cortex and routed via the pyramidal motor system. In contrast, involuntary smiles arise mainly from subcortical nuclei and are routed via the extrapyramidal motor system. Clinical evidence from Parkinson's patients displaying the “masked face” syndrome suggests that the basal ganglia is involved in the production of emotional expression, while evidence from patients with brain lesions exhibiting emotional facial paresis suggests the possible involvement of various regions of the basal ganglia and thalamus. Furthermore, neuroimaging studies have also substantiated the involvement of the basal ganglia. A related observation has been that voluntarily producing and holding an expression normally seen with genuine spontaneous response can induce the corresponding emotion. This effect has been linked to afferent facial feedback received as a result of the facial movements. The induction of emotion is more effective when a person pays specific attention to voluntarily activating muscles that are usually only used involuntarily (e.g., the Duchenne marker), possibly because the voluntary facial expression is then closer to a genuine one.
Functional electrical stimulation (FES) is a technique in which muscles are electrically stimulated by attaching at least two electrodes of an apparatus to a body region. Briefly, functional electrical stimulation is a technique that uses electrical currents to activate nerves connected to the central nervous system which innervate corresponding muscles to a level sufficient to elicit the motor threshold. In other words, in the practice of FES the electrical current is applied at a level to produce muscular movement of a target muscle. FES has been used to activate nerves which innervate extremities affected by paralysis resulting from spinal cord injury (SCI), head injury, stroke and other neurological disorders. FES is also sometimes referred to as neuromuscular electrical stimulation or NMES.
In the practice of FES, at least two electrodes, an anode and a cathode (complementary electrodes), are coupled to the body of an individual. The anode is placed at a region on the body to receive electrical current. Typically, the anode is placed in the vicinity of a peripheral nerve that is innervating the muscle or muscles of interest, for example termed the targeted muscle, and the cathode is placed at a convenient location to close the electrical circuit or loop such that the current can circulate the loop (a complete circuit). Furthermore, in FES, it is preferred that the current applied to the loop is not sufficient so as to cause contractions in the muscles which are not of interest, in other words the electrode pairs should be placed to ensure specificity, i.e., that only muscles of interest are contracted. Various types of electrodes are known, including surface electrodes or implantable electrodes. Furthermore, the stimulation systems may include a multichannel stimulator wherein a plurality of electrodes may be controlled to stimulate one or more muscles or regions of muscles and can be triggered by a switch or several switches or signals from one or more sensors or types of sensors. Fully implantable systems are known which may similarly provide stimulation that can be controlled by switches or sensory signals. Such FES systems may also be preprogrammed to provide the electrical stimulation in a predetermined sequence in open- or closed-loop stimulation patterns.
FES has been shown to have therapeutic applications: artificially stimulating paralyzed or weakened muscles after spinal cord injury or stroke while the individual attempts to voluntarily contract those same muscles can lead to significant functional improvements. Recent studies have shown that this process is accompanied by plasticity in the central nervous system (CNS), with regions affected by the injury and associated with the stimulated muscles displaying increased activity.
A recent pilot study used electrical stimulation of the trigeminal nerve for the treatment of depression where the electrical stimulation was delivered through the skin surface, but not to the level of functional electrical stimulation. In this study, the electrical stimulation was applied so as not to generate muscle contraction or muscle movement, but instead to stimulate a peripheral nerve to evoke sensation or to neuro-modulation of the central nervous system. Furthermore, the trigeminal nerve stimulation was not accompanied by any voluntary movement.
Numerous previous studies have demonstrated the close link between emotion and facial expression. This link is well supported by neuroimaging studies and clinical evidence. It would be desirable to alter the mood or a mood aspect of an individual by exploiting the link between emotion and facial expressions through the use of FES to modulate the neural pathways underlying a given emotion. For example, rather than alter the activity of the neural circuits directly responsible for MDD, which are widespread and not fully understood, an approach may be to directly modulate a mood aspect or parameter of an individual. It would be desirable to use functional electrical stimulation as a means to stimulate facial muscles and by doing so generate desired facial expressions associated with genuine emotions so as to evoke such emotions in the central nervous system and consequently to alter a given mood parameter of an individual so as to alter the overall mood of the individual.
Deep-brain stimulation (DBS) and transcranial magnetic stimulation (TMS) are other electrical modalities currently being explored for the treatment of psychiatric disorders. The application of FES to facial muscles to alter the overall mood of an individual is appealing from a standpoint that it is simultaneously non-invasive (unlike DBS) and precisely targeted (unlike TMS); and, unlike prior uses of electrical stimulation can be employed to activate specific muscles or muscle groups. Therefore FES can be used to generate a very specific facial expression representative of a mood that is desired to be evoked in an individual. The central nervous system (CNS) and muscles are a very tightly coupled system and therefore the emotional state of the CNS of an individual may be reflected outwardly in the muscle contraction levels/activity associated with emotion. Similarly, changes in the activity of various muscles may influence the state of the CNS. By artificially generating facial expressions that are tightly coupled with particular emotions, the emotional state of the individual may be altered.